Centrifugal blasting machines are commonly used for stripping or cleaning floors, storage tank walls, metal castings, and other items. Some machines employ a throwing wheel assembly comprising a plurality of throwing blades mounted on only the front face of a single plate-type rotor, runner head, or other single plate wheel element. Other designs utilize wheels having a pair of opposing plates (i.e., a front plate and a back plate) between which the throwing blades are mounted. Although the single plate and double plate designs all have limited service lives, the single plate-type wheel designs are preferred by many in the art because they are simpler and much less costly to manufacture, balance, and replace.
The blasting machine throwing blades are adapted for receiving a stream of steel shot, hardened steel grit, other abrasive material, or a combination thereof and throwing the abrasive material radially outward from the wheel at an appropriate discharge point. Different treatment profiles can be provided by varying the type or mix ratio of the abrasive material, varying the abrasive size, and/or adjusting the rotating speed of the throwing wheel assembly. The abrasive material cleans the surface of the floor, wall or other item and typically rebounds, along with removed contaminants, dust, and debris, into a separator or other recovery chamber. Unfortunately, because of the action of the abrasive material within the centrifugal blasting apparatus, the throwing blades undergo considerable wear and must be replaced periodically.
In one common type of blasting assembly, the throwing wheel, throwing blades, and the rotating central impeller of the device are all cast in place as a unitary structure. Although such unitary cast structures do not require further assembly and do not present blade retention concerns during installation, they have significant shortcomings and disadvantages. For example, although the throwing wheel and other portions of the unitary cast structure are less susceptible to wear and damage than are the throwing blades, the entire cast structure must be replaced when the blades are worn, or if even just a single blade is chipped or broken. In addition, the unitary casting procedure does not allow the blade portion of the casting to be formed from a different, harder, more wear-resistant material. Further, the use of a harder, more wear-resistant material for forming the entire structure is not practical due to both higher cost and the poor casting characteristics of these harder materials which, for example, make them much more difficult to machine after heat treatment.
To address these issues, and to obtain the benefits of a single plate design, various efforts have been made to develop throwing assemblies wherein the throwing blades are removably mounted on the forward face of a single plate-type throwing wheel. One such throwing wheel assembly is described in U.S. Pat. No. 7,311,584, the entire disclosure of which is incorporated by reference. The prior art assembly comprises; a single plate throwing wheel; a hub affixed to the back of the throwing wheel; a plurality of throwing blades which are removably mount on, and are perpendicular to, the forward face of the throwing wheel; and an impeller centrally mounted on the hub between the inlet ends of the throwing blades. The blades generally extend radially away from the impeller.
As is generally the case, the impeller rotates with the throwing wheel for receiving a stream of abrasive blasting material from a spout and feeding the abrasive material to the throwing blades. The rotating impeller delivers the abrasive material through the discharge opening of a stationary control cage within which the impeller is rotatably positioned. The abrasive material is received on the inlet ends of the blades as the blades rotate past the cage opening. The abrasive material then moves outwardly along the throwing surfaces of the blades and is thrown from the distal ends of the blade throwing surfaces at a desired discharge point.
In the single plate throwing assembly described in U.S. Pat. No. 7,311,584, each of the throwing blades has a holding structure which projects laterally outward from the lower end of the rearwardly facing side edge of the blade. This lower holding structure is configured for removable, locking engagement with a corresponding retaining slot formed in the wheel face. Consequently, during operation, the centrifugal force exerted by the rotation of the throwing wheel operates to continuously urge the blades radially outward into secure engagement with the holding slots formed in the wheel face. However, in removable blade systems such as this when the system is being assembled and installed and there is no centrifugal force acting against the blades, additional retaining mechanisms are needed to at least temporarily hold the blades in place so that they do not fall off the wheel face.
To deal with this problem, U.S. Pat. No. 7,311,584 discloses the addition of a second (upper) holding structure to the side edge of each blade. These upper holding structures are received in peripheral detents formed around the throwing wheel wherein they are contacted with individual biasing springs. The biasing springs operate to continuously urge the throwing blades radially outward such that the lower blade holding structures are always held in locked engagement in the blade retaining slots.
As will be understood by those in the art, a continuing need exists for improved single plate rotating wheel assemblies having removable throwing blades. A need particularly exists for improved assemblies of this type which (a) are lower in cost, (b) are less complicated to assemble, install, and remove, (c) are easier to manufacture, and (d) provide a much simpler yet highly effective means for retaining the removable blades on the throwing wheel during assembly and installation procedures.